Mirrors TikTok's own POST /file/image/ad/upload/.

• Body is forwarded exactly as sent.
• Write — changes a live ad account immediately; needs a write-scoped token.

Uploading a file. As well as the JSON body above, this route accepts multipart/form-data with a file part (other parts ride along as platform fields), or file_base64 inside the JSON body — raw base64 or a data: URI, plus optional file_name/file_type. file_base64 is the only way an MCP caller can send bytes. JPEG, PNG, GIF or WebP, up to 10MB. A rejected type or size is 400 FILE_UPLOAD_INVALID; unavailable storage is 503 FILE_STAGING_FAILED (retry the identical request). Do not send upload_type: "UPLOAD_BY_FILE" — that is TikTok's multipart variant, which this API does not speak; a staged file is sent as UPLOAD_BY_URL.